Help with rigging 3x3 bones and root in blender
Here is a basic solution for this kind of armature:

Extrude a bone from the lower leg, unparent it, it will be the IK target, which will control the lower leg (IK bone). Duplicate the target to make the IK pole target (the bone that will give direction to the knee), parent it to the IK target.



In Pose mode give the lower leg an Ik constraint, assign it the target and pole target, give it a Pole Angle of 180° if necessary and a Chain Length of 3.



To give your bones some custom shapes, in Object mode create an object with the shape you want (example: a circle), then back to the armature, in Pose mode, select the bone you want to shape, in the Properties panel > Bone > Viewport Display > Custom Object > select the circle.